The thieves removed more than 20 square feet of bark from an old oak tree in Danemead wood, near Hoddesdon, Hertfordshire, and removed a whole colony of the caterpillars living underneath.
They probably represented the total population of goat moths in the wood, according to one of Britain's leading entomologists, Colin Plant, who discovered the theft. They had almost certainly been taken by specialist collectors, he said.
The thieves were breaking the law because they had taken the caterpillars from a Site of Special Scientific Interest and did not have the permission of the landowner, the Herts and Middlesex Wildlife Trust.
"This moth is an integral part of the wildlife of this reserve," the trust's conservation manager, Graham White, said. "The thoughtless actions of these thieves ruin reserves for wildlife and for all those who love visiting them."
The goat moth, Cossus cossus, is one of the biggest British moths: the larger brown and buff female has a wingspan of nearly four inches, while the fleshy red caterpillars themselves can be even bigger, at nearly five inches long. They feed on wood by burrowing into the trunks of trees and were once regarded as a pest in orchards, but are now very scarce.
Goat moths belong to a worldwide family of woodboring moths (the Cossidae) whose caterpillars are sometimes known as carpenterworms; in Australia some of them are eaten by Aborigines, who refer to them as "witchety grubs".
The species gets its name from the strong goat-like smell the caterpillars give off. Some collectors claim to be able to detect colonies by smell alone.
Mr Plant, who is editor of the The Entomological Record, discovered the traces of the stolen goat moth colony when the trust commissioned him to survey the moths and butterflies in the wood. He found a recently stripped oak tree and the tell-tale "galleries" - remains of the tunnels under the bark that goat moth caterpillars make.
"Whoever did it used a chisel and stripped the bark off the whole bottom section of an old oak tree four feet wide," Mr Plant said. `They must have taken off more than 20 square feet. Goat moths tend to collect on a single tree so they probably took all the larvae that the wood contained.
"I have no problem with people collecting caterpillars in general, but I do have a problem with people collecting to excess, and collecting in a nature reserve without permission."